European haplogroup U subdivided into two major subhaplogroups, U5 and U1811.

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To further investigate the genotypic basis for the phenotypic differences between U5 and U1811 spermatozoa, we characterized the potentially functional variants in subbranches of U5 and U1811. U5 subdivides into two primary branches: U5a and U5b (Achilli et al., 2005) (Fig. 1). U5a was founded by one cytb missense mutation: nt 14793G/H16R (CI = 54%) and the most abundant U5a sublineage, U5a1 has another cytb mutation nt 15218G/T158A (CI = 79%). The remaining U5 mtDNAs occupy a separate branch, U5b defined by two synonymous variants.

The U1811 lineage divides into several sublineages, including U4, Uk [previously defined as K haplogroup, (Torroni et al., 1996) and U1811rest, the later incorporating several small lineages. The U4 sublineage is founded by four synonymous variants and a cytb missense mutation at nt 15693C/M316T (CI = 74%). The Uk sublineage is founded by four synonymous variants and two non-synonymous mutations: a cytb mutation at nt 14798C/F18L (CI = 77%) and an ATP6 mutation at nt 9055A/A177T (CI = 85%). The U1811rest sublineages lack highly conserved nodal missense mutations.

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The sublineages of U1811 were all lower than the U5 lineages, again confirming that the COIII nt 9477G mutation has a positive effect on ATP production. Within the U1811 sublineages, the rank order of sperm motility and vitality and therefore probable mitochondrial ATP production was Uk, U1811rest, and U4. U4 which harbours the cytb nt 15693C mutation had the lowest sperm motility and vitality, between 4% and 10% below those of Uk and U1811rest (Fig. 3).

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Data from the literature was compiled on the European distribution of 1802 individuals harbouring haplogroup U mtDNAs (Table 2). This revealed a highly significant north–south geographic distribution of different haplogroup U sublineages (χ2 = 109.6, df = 5, P ≤ 0.001). For the U5 subhaplogroup, the U5a sublineage with cytb mutations was more prevalent in northern Europe (ratio = 1.9), while the U5b sublineage without cytb mutations was more common in southern Europe (ratio = 0.7). Of the U1811 subhaplogroup, Uk and U1811rest were equally distributed in both latitudes (ratio = 0.9 and 1.0, respectively), but the U4 sublineage was enriched in the north (ratio = 1.9). Moreover, the frequency of this subhaplogroup, as well as its proportion in the haplogroup U, increased eastwards, reaching maximum values in the populations of Northwest Siberia (Malyarchuk, 2004). Thus the combination of the cytb mutation-containing sublineages U4, Uk and U5a was in substantial excess in northern Europe.

We had previously shown that sperm from men harbouring haplogroup T mtDNAs swim less vigorously than those from haplogroup H. However, the biochemical basis of this motility was difficult to investigate because of the multiple mutations, the most important of which affected respiratory complex I for which there is no crystal structure. To more thoroughly study the relationship between mtDNA variation and differences in mitochondrial energy metabolism, we turned to the analysis of sperm baring haplogroup U mtDNAs. Haplogroup U is a monophyletic ancient and thus heterogeneous maternal lineage that is broadly distributed among European individuals. Several sublineages of haplogroup U were found to be associated with differences in sperm motility and vitality. These differences could be related to a highly conserved missense mutation in the mtDNA COIII gene (V91) and several equally conserved mutations in the cytochrome b (cytb) gene. Moreover, the lineages with the cytb mutations were substantially enriched in northern Europe, while those lacking these mutations were more prevalent in southern Europe. We suggest that some of these ancient conserved cytb missense mutations permitted our ancestors to adapt to cold by partially uncoupling mitochondrial oxidative phosphorylation (OXPHOS).

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